The scale on a map is 7cm : 10km. If two cities are 21cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 7cm on the map corresponds to an actual distance of 10km. A distance of 21cm on the map is the same as 3 $\cdot$ 7cm. The actual distance between the two cities is 3 $\cdot$ 10km, or 30km.